Apex domains cannot have CNAME records.
Question and response
Question
dig slndns.co.uk. NS
Answer
slndns.co.uk. 5 NS ns-fh.ui-dns.org.
slndns.co.uk. 5 NS ns-fh.ui-dns.com.
slndns.co.uk. 5 NS ns-fh.ui-dns.co.uk.
slndns.co.uk. 5 NS ns-fh.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0